The intestine is considered as the the largest immune organ in the body.
The intestinal structure is crucial for the health and performance of animals. In addition to the digestive functions of the gastrointestinal tract (GIT), the immune cells and lymphoid structures present in the GIT constitute the largest immune organ in the body.
The foundations of the mucosal-associated immune system in the intestine are divided into five spheres, which are:
Epithelial barrier, a physical barrier composed of epithelial cell layers (enterocytes and colonocytes), mucus-producing goblet cells (goblet cells), bactericidal substance-producing Paneth cells (defensins), carrier cells (M cells), and intraepithelial T lymphocytes (Figure 1).
Isolated lymphoid follicles, site of initiation of adaptive immune responses, organized structure of immune cell populations.
Peyer’s patches, the site of initiation of adaptive immune responses, an organized structure of multiple lymphoid follicles with germinal centers.
Diffuse lymphoid tissue, macrophages, dendritic cells, mast cells, irritating T lymphocytes, regulatory T cells, IgA-secreting plasma cells scattered within the lamina propia of the mucosa.
Mesenteric lymph nodes, the site of initiation of adaptive immune responses against intestinal antigens brought by the lymphatic system (Gonçalves et al., 2016).
Figure 1. Structure of defense of the intestinal epithelial barrier (adapted from D. Artis. 2008; 8:411 Nature Reviews Immunology).
Intestinal surface
The surface of the mucosa (Figure 2) is covered with mucus formed by mucins, which are secreted by goblet cells and create a barrier that prevents larger particles, including most bacteria, from coming into direct contact with the layer of epithelial cells (Turner, 2009).
Mucins contain different oligosaccharides and include secreted and cell surface glycoproteins. The secreted mucins, including MUC2, MUC5, and MUC6, form a hydrated gel layer that is 300 to 700 μm thick and consists of two layers: an outer less dense layer that is typically colonized by bacteria, and an inner dense layer that is attached to the epithelium and is free of bacteria.
Figure 2. Representation of the intestinal mucosal epithelium.
Paneth cells (Figure 3), located in the crypts of the small intestine, secrete α-defensins. In the colon, β-defensin peptides are produced by absorptive epithelial cells in intestinal crypts, some constitutively and others in respo...
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